1. Field of the Invention
The invention relates to the field of semiconductor light sensors which detect light intensity by producing a current which is thereafter converted into a voltage, and to devices using these sensors. More particularly, it relates to a semiconductor light sensor having particular use in automatic focussing cameras or in photoelectric transfer elements used in manuscript read-out devices, such as are installed in facsimile machines.
2. Background Information
Semiconductor light sensors are particularly useful in applications requiring compact size and/or low energy consumption. Manuscript read-out devices conventionally used in facsimile machines are of two system types: a reduction type, in which images from the manuscript are reduced and formed; and a contact type, in which manuscript images are read in their original size.
Contact-type read-out devices have been developed in recent years because they permit a machine to be made more compact and its light system easier to adjust. Light sensors used in these devices include thin film amorphous silicon, CdS-Se, monosilicon crystals of the charge-coupling device (CCD) type, and MOS type sensors. Sensors employing monosilicon crystals use high-performance elements such as photodiodes and phototransistors as photoelectric elements. A MOS-type light sensor using phototransistors can attain high performance at a relatively low cost, and is widely used in practice.
FIG. 14 shows an example of a conventional MOS-type light sensor device. This device comprises many phototransistor sensors (20.1-20.n), each phototransistor having a base-collector capacitance (22.1-22.n), switches (21.1-21.n) to read out signals from each sensor on an output line, and a reset switch (24) to reset each sensor and the device output. When the reset switch (24) and the switches (21.1-21.n) are turned on (closed), each sensor is reset. In this condition, the base-collector capacitance (22.1-22.n) of each sensor builds up a reverse bias voltage. After resetting, the switches are turned off (open). When the phototransistor sensors (20.1-20.n) are exposed to light, an electric charge develops according to the intensity of light impinging therefrom, and the charge is retained in the base-collector capacitances (22.1-22.n). Then, the switches (21.1-21.n) are turned on and off (closed and opened) sequentially according to the signal (25) received from a scanning circuit to read-out each sensor in turn. Since the base-collector capacitances (22.1-22.n) are recharged at this time, a current flows between the base and emitter. Moreover, a current multiplied by h.sub.fe (common emitter current gain) flows between the emitter and collector. Thus, the light detected by each sensor is converted into a current which is amplified and appears on the output line.
The above-described light sensor device using phototransistors provides an output with high sensitivity as a result of the amplification by the phototransistors. However, as shown in FIG. 15, it becomes difficult to maintain a flow of the base-to-emitter current IBE if the potential difference VBE between the base and emitter is small when the phototransistors deliver their output. Therefore, a current corresponding to the charge discharged from the base-collector capacitance (22) cannot flow between the base and emitter during a read-out time of from several hundred nano-seconds to several micro-seconds if the light intensity incident on the sensor is small. The output of the light sensor decreases sharply in the region of low incident light intensity as shown in section IX of FIG. 16. The linearity of the photoelectric transfer characteristic is lost making it difficult to obtain a high-sensitivity phototransistor sensor.
It is possible to maintain the linearity in the photoelectric transfer characteristics by prolonging the read-out time. However, this in turn increases the time required to read a manuscript, thereby reducing overall device speed.